Examination of Harmonic Analysis Methods Using Semi-regular Sampling Data from an ADCP Installed on a Regular Ferry: Evaluation of Tidal and Residual Currents in the Eastern Mouth of the Tsugaru Strait
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概要
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Some harmonic analysis methods using actual current data measured by an ADCP installed on a regular ferryboat were examined based on the suggestions made by Kuroda and Isoda (2004) regarding the harmonic analysis of annual variations. Hence the tidal and residual currents (seasonal variation) were estimated around the eastern mouth of the Tsugaru Strait. The biggest problem in the harmonic analysis is that the K<SUB>1</SUB>, P<SUB>1</SUB>, and Sa constituents cannot be separated simultaneously since a sun-synchronous infrequent semi-regular sampling causes K<SUB>1</SUB> and P<SUB>1</SUB> aliasing, the period of which is the same as that of the Sa constituent, i.e. 365.24 (d). First, harmonic analysis of the Sa, K<SUB>1</SUB>, O<SUB>1</SUB>, M<SUB>2</SUB>, Q<SUB>1</SUB>, and N<SUB>2</SUB> constituents could spatially estimate the systematic and dominant K<SUB>1</SUB> tidal current. However, the guessed maximum P<SUB>1</SUB> amplitude that may have been included in the residual current was not sufficiently less than the amplitude of the seasonal variation. Therefore, it was impossible to discuss the seasonal variations unless the P<SUB>1</SUB> constituent was estimated, based on assumptions and subsequently excluded. Another harmonic analysis of tidal currents was carried out using the amplitude ratio and phase difference of the P<SUB>1</SUB> tide relative to the K<SUB>1</SUB> tide. The harmonic constants were compared with those estimated from the mooring current measurements made in past years, and the points of commonality and difference between them were clarified. Thus, for the first time, the seasonal variations in the surface currents at the Tsugaru Strait were demonstrated by excluding the estimated tidal currents.
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